The invention relates generally to spacecraft attitude determination and control systems. More specifically, the invention concerns attitude control of a spacecraft orbiting a celestial body in the presence of a plurality of known remote sources of radiated energy.
Studies relating to attitude control for low cost communications satellites having electronically steerable or so-called "agile" beam antennas have concluded that gravity gradient stabilization was the preferred method of attitude control. More accurate active control approaches with unspecified sensing, if any, have required various complex gear, such as pitch wheels for roll/yaw stabilization. Also, more versatile deployment methods for gravity-gradient stabilized spacecraft involving magnetic torquing have been rejected because of the perceived need for magnetometers to properly commutate the magnetic torque generating elements.
The disclosed method of the invention using agile beam sensing for attitude control provides accurate, responsive, wide angle sensing that enables full and continuous attitude determination without the need for a multiplicity of separated dedicated celestial body or inertial sensors. One embodiment of the invention provides continuous yaw rotation and thermal control by means of steady orbit rate precession of the spin axis of a single-body spinning spacecraft whose angular momentum is substantially in the oribt plane. The invention further contemplates associated magnetic torque control laws for minimizing nadir pointing errors using on-board geomagnetic field modeling derived from an agile beam-sensed state estimation.
The invention recognizes that the continuous and complete satellite attitude state estimates which are computed for agile beam formation for communications have the inherent accuracy and bandwidth which are in excess of the satellite attitude stabilization and control requirements, thus obviating separate attitude sensors for use in achieving attitude control. Additionally, the on-board continuous attitude and orbit state estimates based on agile beam sensing enable either deterministic or estimated magnetic field modeling for accurate magnetic torquing without resort to magnetometers.
It is a feature of the invention that the spacecraft agile beam communications antenna is used for determination of the spacecraft attitude.
It is another feature of this invention that the active yaw rotation attitude control law utilized yields a more uniform thermal exposure of the spacecraft components than provided with previous orbiting satellite orientations.
It is a further feature of the invention that the active maintenance of constant nadir pointing of the spacecraft spin axis and active transverse rate damping as contemplated by this invention avoids the pointing performance versus stability-trade off encountered in prior art systems, such as those utilizing passive gravity-gradient stabilization and passive magnetic libration damping.
It is still a further feature of this invention that deployment of the orbiting spacecraft will not require expulsion of propellant mass to achieve the initial spacecraft deployment rates, or deployment of mechanical appendages to augment inertia properties. The lightly damped large angle capture transients as for example, with prior art gravity gradient deployment methods are also provided. Steady nadir pointing can be achieved within one quarter orbit from separation from a spinning dispenser in a nominal injection attitude along the satellite orbit velocity direction.
It is yet a further feature of the invention that the spacecraft design is compatible with nested spacecraft designs, such as those contemplated by the invention disclosed in the above-cited co-pending related patent application.